SOURCE

(function(){
  var _jQuery = window.jQuery,
      _$ = window.$;
   //==========================0.0.1=====================================
  // var version = '0.0.1',
  //     jQuery = function(selector){
  //       console.log(document.querySelector(selector))
  //     };
  // jQuery.prototype={
  //   jquery:version,
  //   constructor:jQuery
  // }
 
  //抽出构造函数
  // function Factory(selector){
  //   return new jQuery(selector);
  // }
  // Factory.fn = jQuery.prototype;
  // Factory.extend = Factory.fn.extend = function(){
  //   console.log(this)
  // }
  // window.$ = window.jQuery = Factory;
  
  // ================================0.0.2================================
  var version = '0.0.2',
    jQuery = function(selector){
    return new jQuery.fn.init(selector)
  };
  jQuery.fn = jQuery.prototype={
    jquery:version,
    constructor:jQuery
  };
  var init = jQuery.fn.init=function(selector){
    if(!selector){
      return;
    }else{
      var elem = document.querySelector(selector);
      if(elem){
        this[0]=elem;
        this.length=1
      }
    }
  }
  init.prototype=jQuery.fn;
  jQuery.extend = jQuery.fn.extend = function(){
    console.log(this)
  }
  
  window.$ = window.jQuery=jQuery;
})()

console.log($.fn.jquery)
console.log($('.box'))

<div class="box">
  
</div>